The present invention relates to a gradation reproduction method for depicting the gradation or tone of an image mainly by the size of dots and to a gradation-reproduced image. More specifically, the present invention relates to a gradation reproduction method used when an image is recorded with an ink-jet printer, a thermal printer, an electrophotographic printer or the like and to a print-output image.
Nowadays, in a printer, various kinds of processing are performed so as to reproduce the gradation of an image smoothly when the image is reproduced.
In an ink-jet printer ejecting ink droplets for image recording, for example, an ejection amount of ink is made constant, meanwhile, the number of dots put on a recording medium is varied in accordance with the gradation of an image, and thus the gradation thereof is reproduced. In such an ink-jet printer, the number of gradation levels (gradients) set in the printer is small. Accordingly, in some cases, the difference between gradation levels becomes unfavorably conspicuous when signal values of the data on an image such as a photographic image are directly converted for image recording and the image is recorded. In order to render the difference between gradation levels inconspicuous, the error diffusion method is used, and the number of gradation levels is increased by using light cyan ink and light magenta ink as well as cyan ink, magenta ink, and yellow ink. Thus, the gradation of an image is depicted smoothly.
The above -mentioned error diffusion method refers to a method, in which when gradation levels of respective pixels are set to the gradation levels proximate thereto, which are set in the printer, setting errors of the gradation level value (gradient) set for a focused pixel as well as of the gradation level values set for the neighboring pixels of the focused pixel are all calculated, and the setting errors of the gradation level values are allotted such that the accumulation of the setting errors of the gradation level values becomes zero.
However, in an ink-jet printer using the error diffusion method, graininess tends to be conspicuous at the middle-level gradation, which is attributed to a random dot arrangement by the error diffusion. Moreover, at the gradation levels on the shadows"" side, a tone jump is apt to occur because dots are blurred and their boundaries become indefinite. Such phenomena have been problematic.
Meanwhile, nowadays, proposed is a recording head using an electrostatic force, that is, a recording head, in which a pulse voltage is applied between a movable electrode provided on a vibration plate and a fixed electrode spaced by a minute distance from the vibration plate and disposed parallel thereto to vibrate the vibration plate, thus ink droplets are ejected.
In accordance with such a recording head, a magnitude of deformation of the vibration plate is changed by changing a pulse width of the pulse voltage to be applied, whereby an ink ejection amount is varied. Thus, the size of the dots put on a recording medium can be controlled. Therefore, unlike a conventional image gradation reproduction mode depicting the gradation levels by the number of dots, a novel image gradation reproduction mode depicting the gradation levels by the size of dots may be conceived. What is expected by use of the novel mode is that the graininess and the tone jump are eliminated without requirement of the error diffusion method, and thus a multi-level gradation (variable constant) and high-resolution image is reproduced.
As one method of depicting the gradation levels by the size of dots, there has been known gradation depiction by use of halftone dots, which has been conventionally used in the printing field. In the gradation depiction by use of halftone dots, the gradation levels from the highlights to the shadows is depicted in, for example, ten levels established in accordance with the dot area ratio while the number of dots is set constant.
However, since the dots representing each level of the gradation are conspicuous, the conventional gradation depiction by use of halftone dots cannot be directly used for the ink-jet printer which is required to output an image with such a multi-level gradation as that of photographic images.
In the field of offset printing, the interval between halftone dots which allows the dots to be inconspicuous is represented by 175 LPI (lines per inch). Consequently, in order to smoothly reproduce the gradation levels from the shadows to the highlights by using one kind of ink, that is, setting the concentration of ink constant and employing the number of halftone dots for the middle-level gradation as the number of halftone dots for all the gradation levels, a recording head with a high resolution of at least about 2400 DPI (dots per inch), corresponding to a dot size of about 10 xcexcm, must be used. However, such a recording head is very expensive. Also in the ink-jet printer, a very expensive recording head with a resolution exceeding 2400 DPI is required, which is not practical. Therefore, it is actually difficult to output a high-quality image with the ink-jet printer to which is applied the conventional gradation depiction by use of halftone dots.
Moreover, also in an electrophotographic printer performing gradation depiction by use of stripes and halftone dots, the dots are conspicuous and the graininess tends to be conspicuous at the highlight gradation levels. These are problems caused by the conventional gradation depiction by use of halftone dots and others.
In a thermal printer of a thermofusion transfer mode, since the gradation depiction by dithering is performed and dither patterns are conspicuous, a high-resolution image cannot be reproduced. This is because the number of gradation levels is small, similarly to the conventional gradation depiction by use of halftone dots.
An object of the present invention is to provide an image reproduction method capable of solving the foregoing problems and recoding an image without use of processing for the gradation reproduction such as the error diffusion method and the dithering, as a gradation reproduction method enabling a smooth gradation depiction to allow dots and graininess to be unobtrusive, and to provide a gradation-reproduced image.
In order to achieve the foregoing object, the present invention provides a gradation reproduction method, comprising the steps of arranging periodically dots changing a size thereof in response to a gradation level when a plurality of gradation levels for depicting an image are reproduced by dot patterns and reproducing the plurality of gradation levels including at least a gradation level having an area coverage modulation ratio of approximately 50% wherein a gradation level having the area coverage modulation ratio of 10% or less is reproduced by dots having a micro density lower than a micro density of dots for reproducing a solid portion.
The micro density means the maximum among the densities of the dots within a minute range, for example, 10xc3x9710 xcexcm dots, which is measured with a microdensitometer and the like. In the case of ink-jet printers, the micro density means the maximum density determined by the optical density of ink. Meanwhile, the solid portion means the portion reproduced by tightly putting dots on a recording medium, namely the darkest portion depictible with dots.
Moreover, an area coverage modulation ratio of X% (X=0 to 100) is represented by the following equation (1), where D100 is the macro density of the solid portion, D0 is the macro density of the portion on which dots are not put (the macro density of the white portion in the case where the recording medium is white paper), and D is the macro density of a focused gradation level. The macro density means the size of a pixel unit to be recorded or the density obtained by averaging densities throughout a certain dot pattern, which are measured with a densitometer by use of an aperture having a diameter of 1 to 5 mm.                               X          ⁡                      (            %            )                          =                                            1              -                              10                                  -                                      (                                          D                      -                                              D                        0                                                              )                                                                                      1              -                              10                                  -                                      (                                                                  D                        100                                            -                                              D                        0                                                              )                                                                                xc3x97          100                                    (        1        )            
In other words, the area coverage modulation ratio is a parameter representing the level of the macro density. The higher the macro density is, the higher the area coverage modulation ratio will be. The area coverage modulation ratio corresponds to the known dot area ratio in the case where the dots are halftone dots.
It is preferable that the gradation reproduction method according to claim 1, wherein the gradation level having the area coverage modulation ratio of 10% or less is reproduced by arranging the dots randomly in an inconstant interval.
Moreover, it is also preferable that the gradation reproduction method according to claim 1, wherein at least one gradation level including a gradation level having the area coverage modulation ratio of approximately 90% is reproduced by periodically arranging dots having a micro density equal to the micro density of the dots for reproducing the solid portion in an approximately constant interval, and by arranging the dots having the micro density lower than the micro density of the dots for reproducing the solid portion on gaps among the dots periodically arranged.
Moreover, it is another preferable that the gradation reproduction method according to claim 1, wherein at least one gradation level including the gradation level having the area coverage modulation ratio of approximately 50% is reproduced by changing orientations of dots in response to dot colors.
Furthermore, it is further preferable that the gradation reproduction method be characterized in that the gradation reproduction method according to claim 1, wherein the dots are formed by ejecting ink from a recording head, and ink having an optical density lower than an optical density of ink for reproducing the solid portion is used when gradation levels are reproduced by the dots having the micro density lower than the micro density of the dots for reproducing the solid portion.
In this case, it is still another preferable that the gradation reproduction method according to claim 5, wherein the ink having the lower optical density is used for the recording head different from the recording head using the ink for reproducing the solid portion.
Moreover, it is still further preferable that the gradation reproduction method according to claim 5, wherein the ink having the low optical density is prepared by diluting the ink for reproducing the solid portion with an ink diluent immediately before image recording.
Moreover, the present invention provides a gradation-reproduced image, comprising dot patterns constituting a plurality of gradation levels of an image, the dot patterns including a dot pattern having dots different in size depending on a gradation level which is formed by being arranged periodically in an approximately constant interval in the plurality of gradation levels including at least a gradation level having an area coverage modulation ratio of approximately 50% and a dot pattern which is formed of dots having a micro density lower than a micro density of dos for reproducing a solid portion in a gradation level having the area coverage modulation ratio of 10% or less.
It is also preferable that the gradation-reproduced image be characterized in that the gradation-reproduced image according to claim 8, wherein a dot pattern having dots arranged randomly in an inconstant interval is formed in the gradation level having the area coverage modulation ration of 10% or less.
Moreover, it is another preferable that the gradation-reproduced image be characterized in that the gradation-reproduced image according to claim 8, wherein dots having a micro density equal to the micro density of the dots for reproducing the solid portion are arranged periodically in the approximately constant interval in at least one gradation level including a gradation level having the area coverage modulation ratio of approximately 90%, and dots having a micro density lower than the micro density of the dots for reproducing the solid portion are disposed on gaps among the dots arranged periodically, thus the dot pattern is formed.
Furthermore, it is further preferable that the gradation-reproduced image be characterized in that the gradation-reproduced image according to claim 8, wherein the dot pattern having dots different in orientation depending on each of colors of the dots is formed in a dot pattern of at least one gradation level including the gradation level having the area coverage modulation ratio of approximately 50%.
xe2x80x9cArranging dots periodically at approximately constant intervalsxe2x80x9d means arranging dots periodically at substantially constant intervals, and is an depiction taking the following into consideration. For example, in the case of a printer using an ink-jet head, the periodical arrangement of dots fluctuates delicately due to a delicate fluctuation of an ejection direction of ink droplets even if the ink droplets are ejected so that the dots can be arranged periodically and if dots cannot be disposed at completely constant intervals due to such mechanical limitations as above and the like, the dots are formed so that the centers of dots may be located at positions shifted by a slight distance (one-fifth of the periodic interval between dots or shorter) from the positions where the dots are to be disposed at constant intervals.